A conservation law that leads to a path-independent integral of fracture mechanics is derived along with the governing equations and boundary conditions for linear piezoelectric materials. A closed-form solution to the antiplane fracture problem is obtained for an unbounded piezoelectric medium. The path-independent integral is evaluated at the crack tip to obtain the energy release rate for a mode III fracture problem. For a fixed value of the mechanical load, it is shown that the crack growth can be either enhanced or retarded depending on the magnitude, the direction, and the type of the applied electrical load. It is also shown that, for certain ratios of the applied electrical load to mechanical load, crack arrestment can be observed.

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The concepts of linear elastic fracture mechanics, generalized to treat piezoelectric effects, are employed to study the influence of the electrical fields on the fracture behavior of piezoelectric materials. The method of distributed dislocations and electric dipoles, already existing in the literature, is used to calculate the electro-elastic fields and the energy-release rate for a finite crack embedded in an infinite piezoelectric medium which is subjected to both mechanical and electric loads. The energy-release rate expressions show that the electric fields generally tend to slow the crack growth. It is shown that the stress intensity factor criterion and the energy-release rate criterion differ when the energetics of the electric field is taken into account. The study of crack tip singular stress field yields a possible explanation for experimentally observed crack skewing in the presence of a strong electric field.

Effect of Pt bottom electrode texture selection on the tetragonality and physical properties of Ba0.8Sr0.2TiO3 thin films produced by pulsed laser deposition J. Appl. Phys. 112, 044105 (2012) Relationship between dielectric coefficient and Urbach tail width of hydrogenated amorphous germanium carbon alloy films Appl. Phys. Lett. 101, 042109 (2012) Interfacial oxide re-growth in thin film metal oxide III-V semiconductor systemsThe contrast mechanisms of domain imaging experiments assisted by atomic force microscope ͑AFM͒ have been investigated by model experiments on nonpiezoelectric ͑silicon oxide͒ and piezoelectric ͓Pb͑Zr,Ti͒O 3 ͔ thin films. The first step was to identify the electrostatic charge effects between the tip, the cantilever, and the sample surface. The second step was to explore the tipsample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (V dc ) applied to the bottom electrode ͑n-type Si wafers͒ for noncontact and contact modes. In addition, a small ac voltage (V ac sin t) was applied to the tip to measure the amplitude (A ) and phase (⌽ ) of the first harmonic ͑͒ signal as a function of V dc . By changing from the noncontact to the contact mode, a repulsive contribution to the static deflection was found in addition to the attractive one and a 180°phase shift in ⌽ was observed. These results imply that in the contact mode the cantilever buckling is induced by the capacitive force between the cantilever and the sample surface. This interaction adds to the tip-sample piezoelectric interaction thereby overlapping the obtained tip vibration signal. Therefore, the antiparallel ferroelectric domain images obtained at zero dc bias voltage will show a variation in A but a negligible one in ⌽ . The capacitive force contribution to the tip vibration signal was further verified in piezoelectric hysteresis loop measurement assisted by the AFM. The observed vertical offset of the loops was explained by the contact potential difference between the cantilever and the bottom electrode. The shape of the curve could be explained by the capacitive force interaction combined with the tip-sample piezoelectric interaction. The experimental results obtained in this study support the interpretation of the cantilever-sample capacitive force contribution to the tip vibration signal in ferroelectric domain imaging experiments using AFM as a probing tool. The use of a large area top electrode between the tip and the sample resulted in the elimination of the electrostatic cantileversample interaction with negligible degradation of the domain contrast. This method proved to be successful because the cantilever-sample interaction was hardly detected and only the tip-sample interaction was observed.

We have demonstrated that ab initio fast folding simulations at 400 K using a GB implicit solvent model with an all-atom based force field can describe the spontaneous formation of nativelike structures for the 36-residue villin headpiece and the 46-residue fragment B of Staphylococcal protein A. An implicit solvent model combined with high-temperature MD makes it possible to perform direct folding simulations of small- to medium-sized proteins by reducing the computational requirements tremendously. In the early stage of folding of the villin headpiece and protein A, initial hydrophobic collapse and rapid formation of helices were found to play important roles. For protein A, the third helix forms first in the early stage of folding and exhibits higher stability. The free energy profiles calculated from the folding simulations suggested that both of the helix-bundle proteins show a two-state thermodynamic behavior and protein A exhibits rather broad native basins.

A screw dislocation in a hexagonal crystal exhibiting piezoelectric behavior is analyzed in the framework of linear elasticity theory. Considered is a straight dislocation with the Burgers vector normal to the isotropic basal plane. In addition to having a discontinuous displacement and a discontinuous electric potential across the slip plane, the dislocation is subjected to a line force and a line charge at the core. The solution is obtained in a closed form by means of a semi-inverse method. The electric enthalpy which takes the place of the internal energy is calculated for the screw dislocation considered in the analysis. The interaction energy for two different internal stress-field systems is derived to calculate the force acting on an electroelastic singularity. Both the standard method and a generalized path-independent integral is used to calculate the force on a piezoelectric screw dislocation subjected to external mechanical and electrical loads. Also calculated are the force between two parallel screw dislocations and the image force due to a free surface.

We proposed a simple van der Waals backbone correction (O2' and OP) to the AMBER ff12 force field in conjunction with the OPC water via an unequal Lorentz-Berthelot combination rule. As tested on four different tetranuceotides such as r(GACC), r(CCCC), r(AAAA), and r(CAAU), this new force field correctly captured each native fold as the largest population. For a RNA tetraloop (UUCG) tested, the stability of its native fold is substantially improved.

Houghton (HG) base pairing plays a central role in the DNA binding of proteins and small ligands. Probing detailed transition mechanism from Watson–Crick (WC) to HG base pair (bp) formation in duplex DNAs is of fundamental importance in terms of revealing intrinsic functions of double helical DNAs beyond their sequence determined functions. We investigated a free energy landscape of a free B-DNA with an adenosine–thymine (A–T) rich sequence to probe its conformational transition pathways from WC to HG base pairing. The free energy landscape was computed with a state-of-art two-dimensional umbrella molecular dynamics simulation at the all-atom level. The present simulation showed that in an isolated duplex DNA, the spontaneous transition from WC to HG bp takes place via multiple pathways. Notably, base flipping into the major and minor grooves was found to play an important role in forming these multiple transition pathways. This finding suggests that naked B-DNA under normal conditions has an inherent ability to form HG bps via spontaneous base opening events.